The present invention generally relates to an implantable cardiac device. The present invention more particularly relates to an implantable cardiac stimulation device and external display for displaying an electrogram which resembles a surface electrocardiogram.
Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as having two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient""s own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves)and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
Implantable cardiac stimulation devices conventionally include an internal telemetry circuit permitting the devices to communicate with an external programmer. The external programmers also include a telemetry circuit with an external antenna or xe2x80x9cwandxe2x80x9d which is held over the implant site to allow the communication between the programmer and the implanted device. With the communication channel thus established, the programmer permits the attending medical personnel to set device operating modes and stimulation and sensing parameters within the device. The communication channel also permits the device to convey to the external programmer operating and sensed physiological data for display. The physiological data may include an intracardiac electrogram (IEGM). The IEGM may be prestored in the device and conveyed to the programmer responsive to a suitable external command from the programmer. The IEGMs are typically stored in response to high rate ventricular events or high rate atrial event triggers. The result is that physicians have more insight into the operation of the devices and have more information about the underlying rhythm that interacts with the device.
In addition to the IEGMs, physicians would like to be provided with a surface electrocardiogram (EKG). Their desire is based upon their day-to-day use of surface EKGs to make diagnosis of arrhythmias. Hence, with both IEGMs and surface EKGs, physicians will have more confidence that they will be able to discern exactly the underlying arrhythmic event that triggered the IEGM storage.
Unfortunately, implantable devices cannot provide surface EKGs. While some programmers of implantable cardiac stimulation systems do accommodate the display of surface EKGs, the surface EKGs available are taken at regular follow-up visits and thus after the arrhythmic event and IEGM storage have occurred. An after the fact surface EKG is not very helpful in support of a diagnosis of a prior arrhythmic episode.
The present invention represents a significant advancement in the provision of heart activity information to support a diagnosis of a prior arrhythmic event. More particularly, the present invention satisfies the need for both IEGMs and surface EKGs taken at the time of an arrhythmic event.
In accordance with broader aspects of the present invention, an implantable cardiac stimulation system and method provides an external display of a heart activity signal sensed internally by an implantable cardiac stimulation device which has the appearance of a surface EKG. The heart activity signal sensed by the implanted device is processed by the device or by the external display or programmer to have frequency characteristics resembling that of surface EKGs.
The system includes at least one implantable electrode that senses the cardiac electrical activity to provide an intracardiac electrogram signal. The intracardiac electrogram signal is high pass filtered with a low frequency cutoff or roll-on of no greater than 0.4 Hz and a high frequency cutoff of no less than 20 Hz. This provides a heart activity signal for display which has the appearance and most attributes of a surface EKG.
The filtering of the IEGM signal may be accomplished within the implanted device or within the external display or programmer. When the filter is provided within the implanted device, the filtered IEGM resembling a surface EKG may be stored within the device for later conveyance to the programmer or conveyed to the programmer in real time for display. When the filter is provided in the external programmer, the IEGM signal to be filtered for display may be the standard IEGM signal provided by the device.
The filter may be implemented in discrete component form or in the form of a digital filter. The digital filter may be an equalizer which enhances low frequency response of the IEGM.
Selective electrode configurations may be provided by the implanted system for providing the IEGM to be processed. The selectable electrode configurations preferably include unipolar atrial, unipolar ventricular or transchamber configurations. The electrode configurations may more specifically include atrial ring and ventricular ring electrodes, atrial tip and ventricular tip electrodes, and ventricular defibrillation electrodes.
Further features and advantages of the present invention may be more readily understood by reference to the following description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a simplified diagram illustrating an implantable stimulation device and lead system embodying the present invention for delivering multi-chamber stimulation and shock therapy;
FIG. 2 is a functional block diagram of a multi-chamber implantable stimulation device embodying the present invention illustrating the basic elements of a stimulation device which can provide cardioversion, defibrillation and pacing stimulation in four chambers of the heart as well as a processed IEGM signal or an IEGM signal to be processed for providing a heart activity signal resembling a surface IEKG in accordance with the present invention;
FIG. 3 is a schematic diagram of a circuit which provides high pass filtering of IEGMs within the device of FIGS. 1 and 2 to provide heart activity signals or IEGMs to be processed or to provide at least one heart activity signal for display which resembles a surface EKG in accordance with the present invention;
FIG. 4 is a chart illustrating the manner in which a first sensing IEGM electrode configuration may be selected from a plurality of possible sensing electrode configurations;
FIG. 5 is a chart illustrating the manner in which a second sensing IEGM electrode configuration may be selected from a second plurality of possible sensing electrode configurations;
FIG. 6 is a simplified block diagram of the elements required in an external programmer for processing an IEGM and displaying the processed IEGM resembling a surface EKG;
FIG. 7 is a graph illustrating the frequency characteristics of a conventional IEGM signal;
FIG. 8 is a block diagram illustrating the filtering stages of a digital filter which may be implemented by the processor of FIG. 6 in accordance with an embodiment of the present invention; and
FIG. 9 is a graph illustrating the frequency characteristics of a processed IEGM which, when displayed, resembles a surface EKG in accordance with the present invention.